Carach Angren

". . . Out from the main range [of Ephel Dúath] there sprang a high barren spur, thrusting eastward like a wall. To meet it there stretched out from the grey and misty northern range of Ered Lithui a long jutting arm; and between the ends there was a narrow gap: Carach Angren, the Isenmouthe, beyond which lay the deep dale of Udûn. . ."

Carach Angren was the pass in the northwest of Mordor, also called by the Mannish name of the Isenmouthe (both versions of the name mean 'iron jaws'). It was formed where spurs reaching out from the ranges of the Ephel Dúath and the Ered Lithui met, leaving only a narrow passage between the Plateau of Gorgoroth and the smaller valley of Udûn to the north.

As the passage to the Black Gate of the Morannon, Carach Angren was heavily fortified, and both the rocky spurs that overlooked it carried fortresses and watchtowers. Across the passage itself, a wall of earth had been built, and a great ditch had been dug across the opening spanned by a single bridge.